This Week in Colorado Weather: August 20, 2024

The Southwest Monsoon will remain active across Colorado this week with the best chances for rainfall coming Wednesday through Friday in the Front Range. Storm chances will be greatest across the Mountains with better forcing, upslope, and unstable air leading to a slight risk of flash flooding there. Temperatures during the week in the Denver Metro area will remain near or above normal, but fortunately nothing too hot. Let’s take a look.

This week’s highlights include:

• Daily Monsoon Storms with a Slight Flood Risk: Rain chances will exist much of the week, but will highest Wednesday through Friday as monsoonal flow is most pronounced during that window. There could even be some flash flooding in higher terrain.
• Temperatures Stay Warm to Hot: Temperatures will stay near or above normal this week with middle 80s to lower 90s, hottest on Tuesday
• Weekend Forecast Looks Okay:  Highs are expected to push back into the 90s for the upcoming weekend alongside somewhat drier conditions. We will need to watch a cold front Sunday which will probably enhance storms late.

DISCLAIMER: This weekly outlook forecast is created Monday morning and covers the entire upcoming week. Accuracy will decrease as the week progresses as this post is NOT updated. To receive daily updated forecasts from our team, among many other perks, subscribe to BoulderCAST Premium.

The late-season monsoon remains active across Colorado

his week started off with a bang in some areas as deep monsoonal moisture combined with moderate shear to produce a round of strong to severe thunderstorms on Monday. These storms contained torrential rain, hail and gusty winds. One particularly strong monsoon thunderstorm blew up over the Flatirons and slammed parts of Boulder with heavy rain and small hail Monday afternoon. The impressive storm can be seen dumping its soggy payload on Boulder in the timelapse below.

Up to 0.75″ of rain fell in just 15 minutes as this storm moved through leading to brief street flooding. The 0.64″ of rain at Boulder’s official station was our largest single-day rainfall in nearly four months (since April 27th)! For some perspective, that day is also the last time we had measurable snowfall in the city! Monday’s storms were generally less intense than this one for the rest of the Denver Metro area.

As expected, as our storms moved eastward late Monday, they grew stronger with numerous reports of large hail, damaging winds, flooding rains and even a few landspout tornadoes across the Eastern Plains. The risk for severe storms will be lower the rest of the week, but torrential rainfall will remain a concern under continued monsoonal flow.

Our primary shower and storm chances this week will be bookended by somewhat drier weather, as the GEFS ensemble plume graphic below shows. This continuation of wet weather is tied to a moist monsoonal pattern, as we’ll discuss below. The rain chances will decrease by the start of the weekend as drier air and a less favorable westerly flow take over. The GEFS shows the most agreement for Front Range rainfall Wednesday and Thursday and we’ll see soon why that is the case. We’ll also discuss the potential for excessive rainfall, with the risk being highest in the Mountains and Foothills.

Tuesday’s weather for us will be mostly dominated by high pressure ridging over northern New Mexico. The ridge axis will extend into Wyoming and Montana. This setup will favor our lowest rain chances of the week, although an isolated storm cannot be ruled out in the higher terrain. Overall, though, Tuesday will be largely dry and see plenty of sunshine with hot temperatures in the low to middle 90s.

Come Wednesday through Friday, the ridge will slide off to the southeast into northern Texas. In addition, a strong trough off the Pacific Northwest will combine with the ridge favoring a deep southwest flow to increase moisture levels across the state of Colorado.

The trough will be to our west through much of the work week, and, as a result, elevated moisture levels will be in place Wednesday, Thursday, and partly Friday too. Precipitable water value on Wednesday will approach ~1.1 inches in the Metro area.

On Thursday, moisture values will increase to 1.2 to 1.3 inches of precipitable water. It would appear the highest rain chances on the Plains would be Wednesday and Thursday, when the moist flow is most favorable for the Front Range. On Friday, we’ll start to see a westerly wind starting to develop, which should commence a downward trend in our rainfall chances.

Over the High Country, rain and storms will exist nearly every day. In fact, a marginal risk of excessive rain (flash flooding) will exist throughout the entire week. Below shows the marginal risk for Thursday, which extends into the Denver Metro area and much of the Rockies. Slow-moving storms with elevated moisture levels will favor moderate to heavy rainfall in localized areas. Recent burn scars will be most at risk.

Estimated rainfall totals through Saturday night are not overly impressive for the Front Range, based on the GFS. It shows anywhere from a tenth to less than a half inch of liquid. However, high-resolution models could bump up these amounts as we get closer. If you look over the higher terrain, though, amounts could range from 0.75 to 1.5 inches. Those amounts would certainly exceed the average rainfall for this period in late-August in the higher terrain.

Temperatures Wednesday through Friday will trend downward as rain chances elevate. Temperatures will be close to 90 degrees on Wednesday will decline to the mid to upper 80s by week’s end. Come Saturday and Sunday, the highest moisture content will spread into the central to northern Great Plains and upper Midwest. That will favor decreasing storm chances by the weekend, and a return to above normal temperatures in the lower 90s.

Here’s a look at the next five days in Boulder and Denver. Temperatures will remain on the warmer side of normal with daily chances for thunderstorms after Tuesday.

The aforementioned Pacific trough will slide eastward Sunday into Monday with a cold front eventually reaching eastern Colorado. Latest model trends suggest that front will arrive late in the day Sunday or Sunday night, with slightly cooler weather developing for early next week.

Enjoy the weather!

Forecast Specifics:

Tuesday: Mostly sunny and toasty with highs in the lower 90s on the Plains and lower 80s in the Foothills. There is a 10% chance of some storms in the late afternoon to early evening, almost exclusively in the higher terrain.

Wednesday – Friday: Partly to mostly cloudy with a 30-50% chance of afternoon/evening showers and storms.  Highs in the upper 80s to lower 90s for the Plains with upper 70s in the Foothills. Rain chances are highest Wednesday and Thursday. Warmest temperatures on Tuesday and lowest Wednesday and Thursday.

Saturday: Lower rain chances with more sunshine and temperatures in the lower 90s for the Plains and lower 80s in the Foothills.

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DISCLAIMER: This weekly outlook forecast is created Monday morning and covers the entire upcoming week. Accuracy will decrease as the week progresses as this post is NOT updated. To receive daily updated forecasts from our team, among many other perks, subscribe to BoulderCAST Premium.

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Andy

Born and raised in St. Louis, Andrew obtained a Ph.D. in Atmospheric Science from the University of Colorado in 2015. From 2015 to 2020, he worked remotely in Boulder as an atmospheric scientist with NOAA's Atlantic Oceanographic and Meteorological Laboratory in Miami. Andy is now a full-time meteorologist.

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